I have pages of handwritten notes that I decided to scan to a PDF. Somehow, 153 pages of notes turned into a 156.4 MB PDF file.

The notes are scanned in color because I used four different colors: black, red, blue, green. In fact, the only colors I really need for the document are those four (plus white). Is there a way to reduce the number of colors in the PDF file? (e.g. via a Unix command?)

Thanks for your response. I looked at ghostscript.com/doc/current/Use.htm and could not find how to set the depth. (I tried the example script with -depth=1 added and that did not make the output black and white.)
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Alan CMay 19 '13 at 1:23